a) Field of the Invention
The present invention relates to a tone signal synthesizer using a delayed feedback type tone signal synthesizing algorithm for synthesizing a tone signal by waveform processing through inputting a driving waveform signal into a closed loop containing a delay means and a filter means and circulating the driving waveform signal in the closed loop. In particular, it relates to a tone signal synthesizer adapted for an electronic musical instrument capable of providing tone control to simulate musical tones of natural musical instruments.
b) Description of the Prior Art
In a waveform read type tone signal synthesizer, tone signals different in pitch are synthesized by reading a fundamental waveform (for example, a sinusoidal waveform) at different reading speed. Because the number of points sampled from the fundamental waveform decreases as the frequency increases, the characteristics of the synthesized tone signals deteriorate. Further, it is difficult to change the signal waveform with the passage of time.
Japanese Patent Postexam. Publication No. Sho-58-58679 has proposed a technique for synthesizing a tone signal by inputting a driving waveform signal into a closed loop formed by a serial connection of a filter and a delay circuit and repeatedly circulating the driving waveform signal in the closed loop. According to this technique, the amplitude, high-frequency content, high-frequency phase, etc. of the signal can be changed widely with the passage of time, so that musical tones more perfectly approaching the musical tones of natural musical instruments can be generated compared with the waveform read type tone signal synthesizer.
In recent electronic musical instruments, a technique for changing various characteristics by touch has been popularized. For example, the attack-decay-sustain-release waveform of a musical tone in a natural musical instrument can be simulated by controlling a sound volume envelope correspondingly to the touch. Further, the vibration of pitch caused by the touch can be simulated.
In natural musical instruments such as a piano, not only a sound volume changes in accordance with touch but the pitch and harmonic structure of a musical tone change just after key depression and then return to the designated pitch and the standard harmonic structure with the passage of time. With respect to the harmonic structure, the musical tone just after key depression contains many non-harmonic pitch components as well as harmonics having pitches expressed by integers or simple fractional numbers with respect to the pitch of a fundamental tone. The non-harmonic pitch components decrease with the passage of time, so that a pure harmonic structure remains.